Carb Tuning/Ignition Timing Experts In Here Please

[BillytheImpaler]

Hello Gents. I've had a nagging problem with my engine for quite some time now and none of my efforts seem to have found a remedy yet. I'm writing here in case one of you can come up with something I haven't thought to consider.

In short, I have preignition/detonation problems with my Pontiac 400. In it I have a relatively mild cam and an AFB-style Edelbrock 750 CFM carburetor. The ignition advance is set to a reasonable amount. It runs about 9.5:1 compression and I always use 92 octane fuel. The engine used to run too hot for my liking but I cured this with a new, gigantic 2-row aluminum radiator and a pair of meaty electric fans on an electric thermostat. I now have about 5k miles on the rebuilt engine.

When it gets up to operating temperature, about 185°F (85 C), it exhibits detonation under partial throttle, between 1/4 and 1/2 open, especially when under load like going up a steep hill. It's fine at idle and, mysteriously, it's also fine at WOT. The spark plugs look great when I've inspected them; a healthy shade of light brown.

I have been able to cure this problem by advancing the timing to about 0°, but then it doesn't run worth a crap; insufficient vacuum, poor idle quality, and seriously reduced performance. I've taken it to three experts, none of whom have been able to exercise its demons. They've all said that it seems to check out OK with the sole exception of the detonation. I even used a boroscope to look inside the combustion chambers for metal chips, piston/head/bore damage, or other forms of scarring that might be creating hot spots. No such luck.

Any ideas? Anything I should check out?

 

[GeX]

I'm not a massive expert with regards to carbs.. or mechnical ignition timing, but run megasquirt on my car so have read a bit in them.

what is the air filter location / air feed like on your car. any chance you can root in any colder air? The fact the engine dets once warmed up is making me think heatsoak. Maybe re-route any water channels in the carb, or intake manfiold. Or install an insulating gasket to stop heat from the head soaking the manifold?

what about octance boosters? I realise the US of A has different Octane markings on pumps (PON vs RON) - but 92 PON is still quite low, I think it equates to roughtly 96 RON. I run 99/100RON in my car.

 

[Dogbreath]

Sound like the distributor advance curve is no longer suited to your engine. Typically a tuned engine requires more static advance, and less total advance so the stock distributor is rarely suitable.

You can get distributor re-curved to suit your engine, though personally I'd throw it in the trash and fit a megajolt or a commercial mapped system which will allow you to get the best out of the engine whilst avoiding any detonation problems.

 

[BillytheImpaler]

For reference most cars run on 87 octane. 92 is the highest you can buy at regular pumps, so I'd prefer to just be able to use that without any additives. Some hot rodders kick this up with Xylene rather than running ridiculously expensive 100+ octane race gas. Aviation fuel frequently has extremely high octane ratings due to its inclusion of tetraethyl lead which has been illegal for cars since the 1970s as it's a pollutant and it destroys mandatory catalytic converters.

With regards to the air intake location, it comes through the classic location, through a round filter box atop the carb. I could attempt to set up a ram air-style intake using ducting from the front or underneath the front. This would require me to fashion a larger air cleaner assembly with space for ducted air to circulate around the filter element before entering for more equal wear.

It just seems that many people are running much larger, hotter, and angrier engines than mine in much smaller, tighter, and more enclosed engine bays than mine and they don't have any problems.

It wouldn't hurt to do, though. It might even give a small power boost, especially at night when it's cooler.

Would it need baffling to keep out rainwater, or would the filter element and the convoluted nature of the ducting provide sufficient protection?

 

[BillytheImpaler]

Dogbreath, it's an aftermarket coil-in-cap GM HEI-style distributor with vacuum advance. I threw out the points mechanical distributor when I rebuilt the motor since I was tired of having to rig up matchbooks and paperclips on the side of the road to replace broken points. I don't really know much about tuning the electronic one though. I know there's an electronic module inside that does something or other.

How do I go about getting it modified? Do I take it to a hot rod shop with a dyno and hire someone to tune it? I don't know what a MegaJolt is and for some reason none of the Google search links are working. Presumably it's a multi-spark ignition box like MSD sells?

 

[penski]

Toluene.

*n

 

[Dogbreath]

Dogbreath, it's an aftermarket coil-in-cap GM HEI-style distributor with vacuum advance.

I'm not very familiar with US engines, but a quick google seems to suggest the GM HEI distributors still use mechanical advance, and that rebuild kits to modify them are available.

Re-curving an HEI distributor

How do I go about getting it modified? Do I take it to a hot rod shop with a dyno and hire someone to tune it?

Idealy you'd want it done on a dyno get get the best ignition advance curve, but it's possible to DIY things if you can get hold of the correct springs (which control the rate of advance). The weights can usually be modified to limit maximum advance by a carefully placed blob of weld.

I don't know what a MegaJolt is and for some reason none of the Google search links are working. Presumably it's a multi-spark ignition box like MSD sells?

It's a DIY igntion controller based on the Ford (hope I haven't offended you) EDIS ignition amplifier and a small controller box. You can program a 2 dimensional map of engine speed/load vs ignition advance, and it has other useful facilities such as soft rev limiter and shift light outputs. Megajolt lite Jr is the official title. It does away with the distributor and uses a trigger wheel mounted on the crank pulley.


EDIT: Just found this place which might be worth a call? $10 seems like a good deal for a range of springs and modified weights. You can also get an adjustable vacuum advance to limit the maximum advance under part throttle which can also be a problem with tuned engines.

 

[BillytheImpaler]

Thanks for that, Dogbreath. I'll check those out.

I do know of a shop not for from here that do dyno testing and tuning. Maybe I can buy some syno time and get the tune sorted for maximum performance from my setup. Now I don't have to worry about it overheating and barfing all over the dyno cell, given the limited airflow.